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Thursday, November 17, 2011

Amplifier Woes -- Help me! Help me!

When I look in the mirror and I see a haunted, obsessed look in my eyes. My wife senses that there is something wrong in the ham shack. She is right. I have an amplifier that wants to be an oscillator. Help me exorcise these gremlins! Guys, this problem is holding up the production of the next SolderSmoke podcast.

My JBOT amp works fine into a dummy load, but when I connect it to an antenna, it gets unstable. Here are some more details of the symptoms:

I am running the JBOT with a 5 element (two toroids, 3 caps) low pass filter (designed by Doug DeMaw and approved by Steve Smith).

With the antenna connected, all is well UNTIL I raise the power out (by varying the input) beyond about 1 watt. Below one watt, the amp is working fine, and it stable. As soon as I hit the 1 watt point, the amplifier seems to break into oscillation. This does not happen into the dummy load.

The antenna is a simple dipole fed by coax. It shows a low SWR. Even when I put an antenna tuner between the amp and the antenna and bring the SWR down to negligible levels, the instability problem persists.

With the amp disconnected from all other circuitry other than the antenna and the power supply, if I just touch the input capacitor, it breaks into oscillation. This does not happen when the amp is working into the dummy load.

I've bolstered the power supply filtering and decoupling. No luck. I tried some de-Qing of the transformers. No luck.

Any suggestions?


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9 comments:

  1. Bill, I know that haunted look...

    I've had RF takeoff at higher powers that was reduced by a good separate earth connection to the rig, power supply and antenna tuner.

    So basically I have lots of RF in the shack when transmitting (it's a wooden shed) and despite a low SWR transmission line it still needs to be shielded from rf being radiated near by.

    ReplyDelete
  2. Hi Bill,

    Since I instigated all this filter stuff, I feel a bit responsible for the problems (but not very much, HA!).

    I'll assume you've attempted to eliminate any RF ground loops(?).

    Have you performed any calculations to ensure your amplifier output impedance is truly 50 Ohms? (yes, I'm reaching here)

    Try placing your SWR indicator between the amp. output and the filter. Anything 'funny' indicated there? You might also try inserting the tuner in the same path to transform any impedance mis-match or to offset reactance showing at the filter input.

    73.......Steve Smith WB6TNL
    "Snort Rosin"

    ReplyDelete
  3. Oh, BTW, the filter I recommended is 'cookbook' GQRP Filter:

    http://www.gqrp.com/technical2.htm

    Those were designed by W3NQN and are 7-elements (3 'L', 4 'C'). Ed 'tweaked' the inductor values so that standard capacitor values can be used without degrading filter performance.

    S.S.

    ReplyDelete
  4. Just one more thing......(remember the old "Columbo" T.V. show?)

    You didn't mention if you're employing a choke on that dipole. If not, maybe RF current flowing on the outside of the coaxial cable shield is causing problems.

    S.S.

    ReplyDelete
  5. Don't forget about shielding! Not only shielding your low-level input of the amplifier, but also the output stage of the transmitter. Keep your higher power RF from leaking out of the output stage - and keep RF fields from invading your low level input.

    ReplyDelete
  6. Just to add to W2AEW's comment, the layout will be critical. Any coupling from output to input will be bad. This is very high gain amp at ~37dB. You may want to try interstage shielding with scrap PCB. Hopefully the layout is "long" with input on one end and output separated on the other end.

    The impact of variations in output impedance is also suspicious. From a transistor's datasheet (if good) a stability factor can be calculated. A long story short is there are impedance regions on the smith chart where instability can be predicted from these parameters(see Linvill and Stern stability factors).

    It is possible any slight mismatch is causing extra radiated signal in the shack an/or coupled to the board and back to the input....just a hunch.

    ReplyDelete
  7. Suggestions:

    Make sure separate RF grounds from transmitter, amp, and power supply are short and low impedance at tx freq.

    Use multi-turn ferrites on all power supply leads including AC.

    Disconnect leads to the solar panel.

    Chip, N5RTF

    ReplyDelete
  8. Hi Bill,

    Is your oscillation LF or HF? If LF I'd suggest putting a 10 ohm resistor in parallel with the RFC in the supply feed to the output stage, along with say a 10uF capacitor on the supply side of that. May or may not help, but good practice anyway.
    72/3; Kevin ZL3KE

    ReplyDelete

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